I did some digging for a short article to come out in next week's New
Scientist.

The fossils actually came from old sulfur mines, and sulfur seeped from
the water into the bone, altering the composition of the organics.
Apparently the adding sulfur slows decay of the organics. It also stiffens
them. The fossils are rigid, not flexible like Mary Schweitzer's T. rex
blood vessels. The bones were intact when fossilized, and the pores were
too small to admit bacterial, although they did let water in. They have
carbon and organics, but they don't have the specific composition yet.
Tests for amino acids and DNA are planned.

I'd bet this had something to do with the preservation environment, but
there's a reasonable chance there are other bones with similar
preservation. They found the fossil marrow structures in about 10% of the
bones they studied. The paper comes out in the August Geology. -- Jeff Hecht

It may seem incredible, but stranger things have happened at
Konservatlagerstätten. In Geiseltal Eocene frog skin was so well preserved
that single epithelial cells were easily visible! In that case preservation
was in coal that accumulated in oxygen-free water that is also thought to
have been very high in CO2.